The present invention relates to an optical emission spectrometer having the features of the preamble of Claim 1 and a method of optical emission spectrometry having the features of the preamble of Claim 9.
The most important example of a spectro-chemical source for sample excitation in the context of emission spectrometry with no doubt is the inductively coupled argon plasma at atmospheric pressure (ICP). The ICP as spectro-chemical source for optical emission spectroscopy (OES) was introduced, in the form of commercially available instruments, in the early 1970s. Depending on the actual spectrometer optics utilized in conjunction with the plasma source, a high analysis efficiency could be achieved, especially by simultaneously measuring many emission lines in the emission spectrum generated by the spectro-chemical plasma source (or even the ‘complete’ emission spectrum), therefore simultaneously determining many or almost all elements present in the sample.
This capability, together with good sensitivity and a large dynamic range allowing to analyze element concentrations from trace to major component level in a wide variety of samples, made optical emission spectrometry with a spectro-chemical plasma source a nearly immediate success, becoming one of the workhorses of inorganic analysis ever since.
The first commercially available ICP-OES instruments employed a vertically oriented ICP, viewed from the side (‘radially’ or ‘side-on’) by the emission spectrometer optics. This configuration allows for good system robustness, allowing analyzing, without much system optimization work, of a multitude of sample types, from aqueous samples, samples with high totally dissolved solids and brines to organic samples, even including highly volatile chemicals. However, due to the limited path length observed in the spectro-chemical source in radial direction, the achievable limits of detection were sometimes inferior to other methods of spectro-chemical analysis, especially compared to graphite furnace atomic absorption spectrometry (GF-AAS).
The latter only changed in the beginning of the 1990s, with the commercial availability of the first ICP-OES instruments employing an axially viewed plasma (‘end-on plasma’), with a horizontal orientation of the spectro-chemical plasma source (ICP). The longer observed path length in the spectro-chemical plasma source axially (‘end-on’) viewed by the spectrometer optics leads to an improvement of the achievable limits of detection by a factor of between ca. 2 and 50, compared to the radial (‘side-on’) plasma observation, also depending on the element to be analyzed and the emission line to be used.
While axially (‘end-on’) viewing the horizontally oriented spectro-chemical plasma source improved the achievable limits of detection, for certain analyte elements (most notably alkaline and earth-alkaline elements), the measurement linearity over the available dynamic range is poorer, compared to radially (‘side-on’) viewing the spectro-chemical plasma source, mainly as result of the so-called ‘easily ionizable elements effect’ (EIEE) shifting the ionization balance in the spectro-chemical plasma source. Occurrence of the EIEE is of course independent from both the spatial orientation of the spectro-chemical plasma source and the observation direction of the optical system utilized, but due to the different optical path lengths employed in radial (‘side on’) or axial (‘end on’) viewing of the plasma source, its influence on the analytical results is only relevant in the axially (‘end-on’) viewing set-up, where it can lead to measurement inaccuracies of up to several hundred percent, for the elements affected by it, if no special pre-cautions, e.g. usage of an ionization buffer, are employed.
A solution to the non-linearities observed in emission spectrometry with an axially (‘end-on’) viewed spectro-chemical plasma source (typically having the source in horizontal orientation) was the addition of a second light path to the optical system, allowing to view the spectro-chemical plasma source also in radial (‘side-on’) direction, sequentially or semi-simultaneously, e.g. by observing emissions of the spectro-chemical plasma source axially (‘end-on’) for one spectral range and radially (‘side-on’) for another one. Such ‘dual observation’ systems allow analyzing elements not affected by the EIEE in axial (‘end-on’) view, with its improved limits of detection, and then (typically sequentially) switching light paths to radial (‘side-on’) observation, analyzing the elements affected by the EIEE with the improved linearity that the radial (‘side-on’) observation of the spectro-chemical plasma source emission is able to provide.
‘Dual observation’ systems of the described kind are known in the prior art e.g. from EP 0 708 324 A2 and are commercially available, utilizing a single, spatially fixed orientation of the spectro-chemical source, relative to the optical system view axis, and two observation light paths, for emission transfer from the spectro-chemical source into the optical system, with these observation light paths oriented orthogonally to the spatially fixed spectro-chemical plasma source, and allowing to view the spectro-chemical plasma source in either axial or radial direction, depending on user choice and analytical requirements, e.g. by employing switchable optical elements in the light paths.
Typically, single orientation dual light-path (‘dual observation’) systems are designed as axially (‘end-on’) viewed systems, often with horizontal orientation of the spectro-chemical plasma source, and having additional radial (‘side-on’) view capability. As an example, attempting applications demanding a vertical orientation of the spectro-chemical plasma source and radial (‘side-on’) observation (e.g. high TDS or organic samples) on such a system typically will, depending on the analytical task at hand, result in compromised performance, usually not comparable to the capabilities of a corresponding single-orientation, single light-path system, which in this case would have a vertically oriented spectro-chemical plasma source viewed in radial (‘side-on’) direction.
Alternatively, ‘dual observation’ systems employing a (fixed) vertical orientation of the spectro-chemical plasma source have become commercially available, but are also expected being limited in typical vertical source/radial view applications, by the required presence of an optical interface or a similar mechanism, allowing to observe the emission of the vertically oriented spectro-chemical plasma source in an axial (‘end-on’) direction.
The article “Evaluation of an Axially and Radially Viewed Inductively Coupled Plasma Using an Échelle Spectrometer With Wavelength Modulation and Second-derivative Detection” (Journal of Analytical Atomic Spectrometry, July 1994, Vol. 9, pages 751-757) by Nakamura et al. discloses the computer controlled movement of the ICP torch assembly between end-on and side-on measurements in a scientific approach. How the movement is carried out can neither be learnt from the description nor the figures.